In various optical systems, aberrations can arise which are unwanted, and which tend to decrease the performance of the optical system.
For instance, optical discs such as CD (Compact Disc), DVD (Digital Video Disc or Digital Versatile Disc), and BD (Blu-ray Disc) are known as information recording media on which information is optically recorded or reproduced. Warped optical discs result in coma aberration in the scanning beam, thus deteriorating the readout signal of the optical recording system. Further, the system may be effected by temperature changes e.g. the objective lens in optical recording systems can give rise to spherical aberration when its temperature is elevated, thus leading to deterioration of the readout signal.
Some optical discs have two or more information layers. When switching from scanning one information layer to another information layer (e.g. from the first to the second layer in a dual layer disc), spherical aberration arises due to the different covering layer thicknesses that have to be traversed by the scanning beam. Such spherical aberration has to be corrected in order to be able to accurately read both information layers.
A known technique for compensating for such unwanted aberrations is to provide a switchable liquid crystal cell within the beam path. The cell is arranged to show a variable amount of spherical aberration, the amount of spherical aberration be controlled by applying a voltage to the cell. EP 1,168,055 describes such an aberration correction device. One disadvantage of such a liquid crystal device is that it requires a relatively expensive liquid crystal cell.
It is an aim of embodiments of the present invention to provide an optical element that addresses one or more problems of the prior art, whether referred to herein or otherwise.
It is an aim of particular embodiments of the present invention to provide a relatively cheap optical element that is suitable for correcting spherical aberration.